Rolling of ships is a well-known risk to ship's safety. It can lead to loss of containers, high stress on the cargo securing system and discomfort for passengers and crew. Rolling of a ship can be described as the periodic movement of a pendulum. The corresponding time period is called the “natural rolling period” of a ship. When a ship is tilted e.g. by a sea wave, the centre of buoyancy of the ship moves laterally and a restoring moment is created which re-stabilizes the ship. Most often, the pendulum-type of rolling will dampen quickly due to the restoring moment and due to friction forces between the ship's hull and water. Rolling may be triggered by environmental disturbances such as waves, wind or sea currents. In some cases, disturbances can have very low energy and still develop successively into a high-amplitude rolling by an energy transfer mechanism between two states. For example a known dangerous variant of a critical rolling is the so called “parametric rolling”, emerging when sea waves along the ship have a period that matches in a certain relation a ship's natural period.
A rolling with maintained amplitude and constant period is called a “critical rolling”.
Historically, different types of devices have been proposed to reduce rolling, such as bilge keels, film stabilizers or rudder movements. However, rolling is still a major problem for ship navigation.